1. Technical Field
The present disclosure generally relates to displacement sensors, and more specifically, to non-contacting displacement sensors.
2. Description of the Related Art
Displacement sensors can be widely used in a number of industrial applications. Conventional displacement sensors have a variety of structures and working principles. For example, conventional displacement sensors include contacting potentiometers (e.g. resistance sensors), inductance sensors (e.g. Linear Variable Differential Transformer (LVDT) sensors and eddy current sensors), and non-contacting potentiometer displacement sensors.
A contacting potentiometer has a simple and low-cost structure. However, because this type of sensor includes a moving wiper that constantly contacts a resistive surface, a contacting potentiometer suffers from a limited life span, poor environmental resistance, and a degradation of the output signal over the sensor's lifetime.
An inductance sensor, on the other hand, is a type of non-contacting sensor, and thus does not suffer from the drawbacks of the contacting potentiometer. The most widely used inductance sensor is an LVDT sensor, which is extremely precise and can have a long useful lifetime. However, to improve linearity, LVDT sensors typically include three electromagnetic coils. These coils can be difficult and costly to manufacture. In addition, the plunger is comprised of ferrite material which exhibits poor resistance to some environments, such as acidic environments.
One embodiment of a non-contacting potentiometer sensor is described on the world-wide web page of Novotechnik of Ostfildern, Germany (http://www.novotechnik.com/). The structure consists of two parallel tracks and a moveable element for capacitively coupling the tracks. The two tracks consist of a first resistive track and a second low ohm collector track. An alternating current (AC) power source supplies a voltage across the resistive track. As the moveable element translates across the two tracks, evaluating electronics pick up a voltage signal from the collector track. However, the device suffers from a coupling capacitance between the collector track and the resistive track, as well as stray capacitance, which corrupts the output signal. Novotechnik apparently sells a device which uses a feedback loop to adjust the supply voltage signal in an attempt to compensate for the coupling capacitance. However, the device can be complicated to manufacture and does not completely account for the interference from stray capacitance and capacitive coupling between the resistive and collective tracks.
Another approach is described in U.S. Pat. No. 5,079,500 (the '500 patent), which describes yet another non-contacting potentiometer circuit arrangement. The potentiometric circuit arrangement is based on a resistive potentiometer track and a capacitively coupled wiper. The resistive track is driven by switched alternating voltages of +/−V and −/+(X−V), where X is a reference voltage and V is the output of an integrator to which the voltage sensed by the wiper, and rectified, is applied. A null voltage point establishes itself at the position of the wiper and stabilizes the integrator output at a direct current (DC) voltage proportional to the distance of the wiper from the track end. Any wiper displacement taps a non-null signal which, integrated, applies new voltages to the track until the null point is re-established at the wiper position. The circuit arrangement partially accounts for signal noise through the use of a feedback loop for altering the input voltage across the track. In addition, a preamplifier attached to the wiper amplifies the signal to mitigate signal noise induced by electromagnetic signals from other objects.
Conventional displacement sensors may be used in conjunction with a hydraulic or pneumatic cylinder to determine the displacement of the piston. However, such use typically comprises a standalone displacement sensor in conjunction with a conventional hydraulic or pneumatic cylinder. Therefore, the resulting cylinder structure can be very complex, causing an associated cost of manufacture to be relatively high. Adding the displacement sensor to the hydraulic or pneumatic cylinder can also increase the size of the cylinder, making them impractical for many applications.
Accordingly, what is needed is a non-contacting sensor structure that can achieve a precise output signal with low noise and without the use of a feedback circuit to account for stray and/or coupling capacitance. Further, a non-contacting sensor structure is needed that is simple in construction and does not need a preamplifier attached to the wiper for mitigating signal noise. Additionally, a non-contacting displacement sensor is needed that can inherently be used as a hydraulic or pneumatic cylinder, advantageously resulting in a hydraulic or pneumatic cylinder having the capability of providing a signal that can be used to determine the position of a piston within the hollow cavity of the cylinder, without a substantial increase in size or complexity from conventional hydraulic or pneumatic cylinders.